1. Field of the Invention
The present invention relates to a surface plasmon resonance device, and particularly to a surface plasmon resonance device suitably used for measuring the dielectric constants of various samples.
2. Description of the Related Art
A conventional known method for measuring the dielectric constant of a surface of a sample is a surface plasmon resonance method (for example, Japanese Unexamined Patent Application Publication No. 11-271215). Among modes of plasmons referring to collective resonance of free electrons present in a conductive material such as a metal or the like, the conventional method uses a surface plasmon mode in which plasmons are localized at a surface of the conductive material. The principle of measurement is as follows: Surface plasmons are coupled with an electromagnetic wave in a dielectric material at an interface between the dielectric material and a conductive material such as a metal or the like. The mode of the electromagnetic wave in the dielectric material changes due to the influence of a change in the dielectric constant of the dielectric material, and the propagation of plasmons in the conductive material such as metal or the like changes with frequencies. Therefore, a change in dielectric constant of a dielectric sample can be read as a change in frequency of the surface plasmon mode.
However, when an interface between a dielectric material and a conductive material such as a metal or the like is planar, an electromagnetic wave propagating in the dielectric material cannot be coupled with the plasmons in the conductive material. This phenomenon will be described below on the basis of a metal as an example of a conductive material. FIG. 12 is a schematic view of a metal-dielectric interface, and FIG. 13 shows a dispersion relation of surface plasmons in the system shown in FIG. 12. In FIG. 13, a component of the wavenumber of the plasmons parallel to the interface is shown as abscissa kx, and the frequency of the plasmons is shown as ordinate ω. The dispersion relation of the surface plasmons is shown by a solid line and represented by the following equation:
      k    x    =            ω      c        ⁢                                        ɛ            d                    ⁢                      ɛ            ⁡                          (              ω              )                                                            ɛ            d                    +                      ɛ            ⁡                          (              ω              )                                          wherein ∈d is the dielectric constant of a dielectric material, and ∈(ω) is the dielectric constant of a metal.
∈(ω) is represented by the following equation:
      ɛ    ⁡          (      ω      )        =            ɛ      ∞        -                  (                              ω            p                    ω                )            2      wherein ∈∞ is a constant, and ωp is the plasma frequency of a bulk metal. The frequency of surface plasmons is saturated at a high wavenumber kx, and asymptotically approaches a dotted line shown by ω=ωsp in FIG. 13, the dotted line being represented by the following equation:
      ω    sp    =            ω      p                                ɛ          ∞                +                  ɛ          d                    Among electromagnetic waves present in the dielectric material, an electromagnetic wave whose propagation direction is parallel to the interface has a dispersion curve shown by a broken line in FIG. 13. This line is in contact with a dispersion curve of surface plasmons at the origin, and is represented by the following equation:
      k    x    =            ω      c        ⁢                  ɛ        d            
The dispersion relation of an electromagnetic wave in a dielectric material is generally represented by the following equation:
            k      x      2        +          k      z      2        =                    (                  ω          c                )            2        ⁢          ɛ      d      wherein kz is a component of the wavenumber of plasmons vertical to the interface. Among electromagnetic waves in the dielectric material, an electromagnetic wave showing a dispersion curve lying above the broken line in FIG. 13 has a real number kz and propagates in the z direction shown in FIG. 12. On the other hand, an electromagnetic wave showing a dispersion curve lying below the broken line in FIG. 13 has a pure imaginary number kz and attenuates in the z direction shown in FIG. 12 to cause so-called evanescent light. Since a wavenumber component kz parallel to the interface is kept by applying a connection condition for an electromagnetic wave at the interface, the electromagnetic wave coupled with surface plasmons in the dielectric material is limited to an electromagnetic wave showing a dispersion relation lying below the broken line in FIG. 13, i.e., evanescent light.
As described above, an electromagnetic wave which can be coupled with surface plasmons is limited to evanescent light attenuating at a metal surface. Therefore, some consideration is required for exciting surface plasmons by optical means. In a method for exciting surface plasmons, a metal is formed in a thin film, and light is incident on the side opposite to the side in contact with a dielectric sample. Therefore, a three-layer structure comprising a dielectric sample, a metal layer, and a transparent medium is generally used for exciting surface plasmons. FIG. 14 is a schematic drawing of this structure. As the transparent medium, for example, glass or plastics, is frequently used because visible light is used in ordinary measurement. The metal layer is thin (for example, about 50 nm) enough to permit visible light incident on one side to reach the other side.
Japanese Unexamined Patent Application Publication No. 6-50883 proposes a surface plasmon resonance measuring device comprising, without using a movable part, a block having a surface which internally reflects an electromagnetic radiation beam transmitted therethrough, a periodic structure layer disposed on the surface of the block, and a conductive layer provided on the periodic structure layer. However, in this document, the periodic structure layer is not described in detail.
However, for a sample having a higher dielectric constant than that of a transparent medium, the dielectric constant cannot be measured by the above-mentioned method. This will be described below. FIGS. 15 and 16 show dispersion relations of surface plasmons appearing in the system shown in FIG. 14. There are two types of surface plasmons corresponding to the two types of interfaces including the metal-dielectric sample interface and the metal-transparent medium interface present in the system shown in FIG. 14, and the surface plasmons are localized at each interface. In FIGS. 15 and 16, the frequencies of both types of surface plasmons are saturated at high wavenumbers kx, and gradually approach the dotted lines (ω=ωspd, ω=ωspg) in FIGS. 15 and 16. The value of each of the dotted lines is as follows:
            ω      sp      d        =                  ω        p                                          ɛ            ∞                    +                      ɛ            d                                ,            ω      sp      g        =                  ω        p                                          ɛ            ∞                    +                      ɛ            g                              wherein ∈g is the dielectric constant of the transparent medium. In each of FIGS. 15 and 16, a broken line is a tangent to a dispersion curve of surface plasmons at the metal-transparent medium interface at the origin. Identically to the above, a dispersion curve of an electromagnetic wave propagating in the transparent medium lies above the broken line. On the other hand, evanescent light in the transparent medium shows a dispersion curve lying below the broken line.
FIG. 15 shows the case in which the dielectric constant ∈d of the dielectric material is lower than the dielectric constant ∈g of the transparent medium. In this case, the following relation is established:ωspd>ωspg Therefore, the dispersion curve of surface plasmons at the metal-dielectric sample interface partially lies above the broken line. Thus, an electromagnetic wave propagating in the transparent medium can be coupled with surface plasmons at the metal-dielectric sample interface. Consequently, the dielectric constant of the sample can be measured by applying light at an appropriate frequency to the transparent medium layer.
On the other hand, FIG. 16 shows the case in which the dielectric constant ∈d of the dielectric is higher than the dielectric constant ∈g of the transparent medium. In this case, the following relation is established:ωspd<ωspg Therefore, the dispersion curve of surface plasmons at the metal-dielectric sample interface entirely lies below the broken line. Thus, an electromagnetic wave propagating in the transparent medium cannot be coupled with surface plasmons at the metal-dielectric sample interface. Consequently, there is the problem that the dielectric constant of the sample cannot be measured with light incident on the transparent medium layer.